Sensitivity of mountain riverine air–water CO2 exchange estimates to elevation decompression and localized atmospheric CO2 variability

Abstract

Mountain rivers act as critical CO₂ outgassing hotspots in the global carbon cycle. However, conventional air–water CO₂ exchange estimates often rely on static atmospheric CO₂ boundary conditions, which introduces potential uncertainties in topographically complex regions. Integrating multi-site spatial surveys across an 1,100 m elevation gradient and a one-year high-frequency in-situ monitoring campaign at a site in the subalpine Jiuzhaigou karst catchment, we decouple the independent effects of physical topography and micro-meteorology on riverine CO₂ evasion. First, altitude-driven barometric decompression steepens the air–water CO₂ partial pressure gradient. While atmospheric CO₂ calculations do not universally assume standard sea-level pressure, failing to apply site-specific local barometric corrections can alter each site’s mean riverine CO₂ exchange rate by up to 23.2 mmol m⁻² h⁻¹, with the absolute magnitudes dependent on local turbulence. Independently, localized atmospheric CO₂ pooling dynamically shifts the instantaneous air–water gradient. This pooling occurs not only at night but also during the daytime, driven by local microclimate, biological processes, and human activities. Second, despite substantial daytime photosynthetic CO₂ drawdowns, the karst waters maintain continuous year-round outgassing. This persistent supersaturation is predominantly sustained by massive, continuous external groundwater and soil dissolved inorganic carbon inputs that consistently exceed evasion and assimilation rates. Ultimately, while boundary simplifications do not significantly impact annual cumulative CO₂ emissions (0.02%−11% deviation), integrating dynamic barometric corrections and localized atmospheric CO₂ monitoring is essential for accurately resolving high-frequency instantaneous outgassing kinetics in complex terrains.